1. Any type of business needs to be registered if there's profit to be made by law.

2. No downside except the RM70-ish yearly renewal fee.

3. Need to decide on the name upon registration if not mistaken. Not interchangeable (although legal & tax-wise you're the same entity).

4. That's up to your internal bookkeeping. Recommend splitting them up from the beginning since it's a hassle to filter out business & non-business transaction of 'shared' account.

5. Yes best to prepare, cause you'll need it for tax declaration at LHDN eventually. Don't need to be audited, but atleast have some resemblance of a proper one.